It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
Hydrogen Peroxide
Hydrogen peroxide (H2O2) is a strong oxidising agent and is considered a highly reactive oxygen species.
Its oxidising capacity is so strong that in concentrated form it is used as a rocket propellant. Its strong oxidising capacity makes it particularly well suited for use as a bleach, cleaning agent and antimicrobial for industrial and domestic use. One of the advantages of hydrogen peroxide is that it is a stronger oxidising agent yet more environmentally acceptable than chlorine based oxidising agents.
The market for hydrogen peroxide is large and continues to expand, for example, from about 1.9 million tonnes in 1994, to 2.2 million tonnes in 2006, to an estimated 4.3 million tonnes 2012. Due to the potential hazardous nature of the processes that involve production, storage and transportation of hydrogen peroxide at high concentration, significant interest recently has been given to the development of alternative production methods, particularly in-situ processes.
Hydrogen peroxide is principally manufactured by the ‘anthraquinone process’ consisting of the autoxidation of a 2-alkyl anthrahydroquinone (or 2-alkyl-9,10-dihydroxyanthracene) to the corresponding 2-alkyl anthraquinone. For example, the cyclic reaction depicted at equation (1) shows the oxidation of 2-ethyl-9,10-dihydroxyanthracene (C16H12(OH)2) to the corresponding 2-ethylanthraquinone (C16H12O2) and hydrogen peroxide.

Most manufacturers use the Riedl-Pfleiderer process, which includes the step of bubbling compressed air through a solution of the anthracene. Oxygen in the air reacts with the labile hydrogen atoms of the hydroxy group, giving hydrogen peroxide and regenerating the anthraquinone. The hydrogen peroxide thus generated is extracted and the anthraquinone derivative is reduced back to the dihydroxy (anthracene) compound using hydrogen gas in the presence of a metal catalyst. The cycle is then repeated.
The overall equation for the process is:H2+O2→H2O2  equation (2)
The economics of the process depend heavily on effective recycling of the quinone (which is expensive), extraction solvents, and the catalyst and many attempts have been made to improve the economics of the process.
For example Solvay have improved productivity and reduced the cost of production by optimising the distribution of isomers of 2-amyl anthraquinone and pursuing economies of scale. This improved process was implemented in 2008 in a “mega-scale” single-train plant in Zandvliet (Belgium) and another in 2011 in Map Ta Phut (Thailand). (Hydrogen Peroxide 07/08-03 Report, ChemSystems, May 2009).
Processes for producing hydrogen peroxide directly from the elements has been of interest for many years. However, one of the problems associated with this approach is that the reaction of hydrogen with oxygen thermodynamically favours production of water. While use of a finely dispersed catalyst is beneficial for promoting selectivity to hydrogen peroxide, the selectivity is still not sufficiently high for commercial development of the process. In an effort to improve the selectivity researchers have developed minute (nanometer-size) phase-controlled noble metal crystal particles on carbon support. Evonik Industries, established a pilot plant in Germany in late 2005 using this catalyst and has claimed that there are reductions in investment cost because the process is simpler and involves less equipment. However, the process has the drawbacks of being more corrosive and unproven and yields low concentrations of hydrogen peroxide (about 5-10 wt % as compared with about 40 wt % via the anthraquinone process).
In 2009, another attempt was made to develop a process for direct synthesis using a gold-palladium nanoparticulate catalyst. (G. J. Hutchings et al, Science 2009, 323, 1037) The catalyst is claimed to have the advantage of reducing hydrogen peroxide decomposition and potentially being an inexpensive, efficient and environmentally friendly process. Hydrogen peroxide tends to decompose spontaneously, and even more rapidly under the influence of the catalysts typically used in direct synthesis. However the use of a gold-palladium nanoparticulate catalyst typically achieves only very low concentrations of hydrogen peroxide (less than about 1-2 wt %).
Attempts have also been made to produce alkaline hydrogen peroxide using a monopolar cell to electrolytically reduce oxygen in a dilute sodium hydroxide solution. (Hydrogen Peroxide 07/08-03 Report, ChemSystems, May 2009).(Anode) 2OH−------------→H2O+½O2+2e−  equation 3(i)(Cathode) H2O+O2+2e−------------→HO2−+OH−  equation 3(ii)(Overall) NaOH+½O2------------→HO2Na  equation 3(iii)
It was shown recently that significantly lower production costs can be achieved in the system where hydrogen and hydrogen peroxide are produced simultaneously by water electrolysis. (Ando, Y. and Tanaka T., ‘Proposal for a new system for simultaneous production of hydrogen and hydrogen peroxide by water electrolysis’, International Journal of Hydrogen Energy, 2004, 29(13), 1349-1354).2O2+2H++2e−→H2O2(E0=0.69 V vs NHE)  equation 4(a)2H2O→HOOH+2H++2e−(E0=1.776 V vs NHE)  equation 4(b)2H2O→O2+4H++4e−(E0=1.23 V vs NHE)  equation 4(c)4H++4e−→2H2(E0=0 V vs NHE)  equation 4(d)
In order for this system to be viable, however, the water oxidation catalyst should promote formation of hydrogen peroxide and inhibit the oxygen evolution reaction (equation 4(c)) which is the more thermodynamically favourable process.
A number of carbon electrodes allowed simultaneous production of hydrogen and hydrogen peroxide using 2V total cell potential. However the efficiencies of this process for hydrogen peroxide production (30-50%) and low current densities are not sufficient for practical use. It is clear that the development of catalyst/electrolyte combination which allows water splitting according to equations 4(b) and 4(d) with high efficiency and low overpotential would allow significant reduction in the energy cost of production for hydrogen and hydrogen peroxide via electrochemical water spitting. Alternatively high efficiency electrolysis according to 4(a) and 4(b) would allow hydrogen peroxide production at both electrodes.
The ability to split water to produce hydrogen simultaneously with hydrogen peroxide is of interest in the field of alternative energy technologies, particularly the use of hydrogen production as the main energy carrier in the proposed “hydrogen economy”. The fundamental processes for producing and converting hydrogen are well-known and the technologies have proven to be practical in large-scale operation. However, current low temperature water electrolysis processes are only 50-62% energy efficient and a cost analysis as part of the United States Council for Automotive Research, Department of Energy (USCAR/DOE) Hydrogen Roadmap suggests that efficiency improvements to 74% are needed in order to meet the DOE cost goal for hydrogen of $2-$3 per ‘gallon of gas equivalent’. The biggest source of inefficiency in these water electrolysis cells is the oxygen generating electrode where substantial over-potentials, typically in excess of 450 mV, are required to generate useful rates of water oxidation. For this reason there has been a major research effort devoted to the development and understanding of novel water oxidation catalysts.
Ideally a water oxidation catalyst should be based on abundant, low cost materials, have high turnover frequencies and remain active over prolonged periods of time. Currently, commercial electrolysers are based on nickel anodes, which require high operational overpotential, and hot alkaline solutions. The most efficient catalysts known are those based on transition metal oxides including, MnOx, Co3O4, RuO2, and IrO2. On the other hand it has recently been suggested that a process for water electrolysis that proceeds via hydrogen peroxide rather than oxygen may be more energy efficient. In smaller scale processes the hydrogen peroxide itself may be of significant value. In large scale fuel process, the hydrogen peroxide could be directly disproportionated into oxygen and water.
Accordingly there is an ongoing need for processes having improved efficiency or economy for production of hydrogen peroxide.
There is also a need for processes that have reduced reliance on consuming energy from non-renewable sources such as fossil fuels.